They first created induced pluripotent stem (iPS) cells which they mixed with other cell types and coaxed into 'liver buds' -- the precursor clusters that develop into a liver.

The buds, each about five millimetres big, were then transplanted onto a mouse brain, where they were observed transforming into a "functional human liver" complete with blood vessels, the scientists wrote.

"To our knowledge, this is the first report demonstrating the generation of a functional human organ from pluripotent stem cells," according to the report.

The technique has yet to be tested in humans, but serves as an important proof of concept, the researchers add.

Reprogramming cells

Stem cells are infant cells that can develop into any part of the body.

Until a few years ago, when iPS cells were created, the only way to obtain stem cells was to harvest them from human embryos.

This is controversial because it requires the destruction of the embryo, a process to which religious conservatives and others object.

iPS cells are easily-obtainable mature cells that are 'reprogrammed' into a versatile, primitive state from where they can develop into any kind of cell in the body.

Takebe told a press conference ahead of the report's release that the human-made liver was observed through a replacement glass skull that was fitted around the mouse's brain.

The liver developed blood vessels which fused with those of the animal.

"We have concluded that this liver is functioning," the scientist ways. "We think this is enough for improving the survival after liver failure."

Promising start, but more work needed

Scientists commenting on the research described it as promising.

Associate Professor Ernst Wolvetang, of the Australian Institute for Bioengineering and Nanotechnology says the research will may one day alleviate the shortage of donated livers for transplantation.

"This provides proof of concept that this may be possible in humans in the future.," says Wolvetang.

"It however remains to be established whether the artificially regrown livers from iPSC will prove to be safe in the long-term.'

Stuart Forbes, professor of transplantation and regenerative medicine at the University of Edinburgh, is equally cautious about the results.

"Whilst the title of the paper is 'functional human liver', these liver buds do not contain the biliary structures (which drain toxins out of the liver) or immune cells that characterise real human liver," he says.

Takebe says the method may also work in organs like the pancreas, kidneys or lungs, but it would be another 10 years before trials are done in humans.

One key requirement would be to shrink the 'buds' to a much smaller size so they can be injected into the bloodstream and taken up by the body internally, he says.

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